The electrophoretic display (EPD or EPID) is a non-emissive device based on the electrophoresis phenomenon of charged pigment particles suspended in a solvent. It was first proposed in 1969. The display typically comprises two plates with electrodes placed opposing each other, separated by spacers. One of the electrodes is usually transparent. A suspension composed of a colored solvent and charged pigment particles, is enclosed between the two plates. When a voltage difference is imposed between the two electrodes, the pigment particles migrate to one side and then either the color of the pigment or the color of the solvent can be seen according to the polarity of the voltage difference.
There are several different types of EPDs. In the partition type EPD (see M. A. Hopper and V. Novotny, IEEE Trans. Electr. Dev., 26(8):1148–1152 (1979)), there are partitions between the two electrodes for dividing the space into smaller cells in order to prevent undesired movements of particles, such as sedimentation. The microcapsule type EPD (as described in U.S. Pat. Nos. 5,961,804 and 5,930,026) has a substantially two dimensional arrangement of microcapsules each having therein an electrophoretic composition of a dielectric fluid and a suspension of charged pigment particles that visually contrast with the dielectric solvent. Another type of EPD (see U.S. Pat. No. 3,612,758) has electrophoretic cells that are formed from parallel line reservoirs. The channel-like electrophoretic cells are covered with, and in electrical contact with, transparent conductors. A layer of transparent glass from which side the panel is viewed overlies the transparent conductors.
An improved EPD technology was disclosed in co-pending applications, U.S. Ser. No. 09/518,488, filed on Mar. 3, 2000 (corresponding to WO 01/67170), U.S. Ser. No. 09/759,212, filed on Jan. 11, 2001 (corresponding to WO 02/56097), U.S. Ser. No. 09/606,654, filed on Jun. 28, 2000 (corresponding to WO 02/01281) and U.S. Ser. No. 09/784,972, filed on Feb. 15, 2001 (corresponding to WO 02/65215), all of which are incorporated herein by reference. The improved EPD comprises closed cells formed from microcups of well-defined shape, size and aspect ratio, filled with charged pigment particles dispersed in a dielectric solvent, and sealed with a polymeric sealing layer.
All of these EPDs may be driven by a passive matrix system. For a typical passive matrix system, there are row electrodes on the top side and column electrodes on the bottom side of the cells. The top row electrodes and the bottom column electrodes are perpendicular to each other. However, there are two well-known problems which are associated with EPDs driven by a passive matrix system: cross-talk and cross-bias. Cross-talk occurs when the particles of a cell (pixel) are biased by the electric field of a neighboring cell (pixel). FIG. 1 provides an example. The bias voltage of the cell A drives the positively charged particles towards the bottom of the cell. Since cell B has no voltage bias, the positively charged particles in cell B are expected to remain at the top of the cell. However, if the two cells, A and B, are close to each other, the top electrode voltage of cell B (30V) and the bottom electrode voltage of cell A (0V) create a cross talk electric field which forces some of the particles in cell B to move downwards. Widening the distance between adjacent cells may reduce such a crosstalk effect but the resolution of the display will also be reduced.
The cross talk problem may be lessened if a cell has a significantly high threshold voltage. The threshold voltage, in the context of the present invention, is defined to be the minimum (or onset) bias voltage required to move particles away from their current position. If the cells have a sufficiently high threshold voltage, the cross-talk may be reduced or eliminated without sacrificing the resolution of the display. A high threshold voltage may be achieved by, for example, increasing the particle-particle interaction or the particle electrode interaction in the electrophoretic cells. Unfortunately, most approaches to increase the threshold voltage tend to result in a significant increase in display driving voltage or a decrease in switching rate.
In addition to the crosstalk by neighboring cells, cross bias is also possible in a passive matrix display. The voltage applied to a column electrode not only provides the driving bias for the cell on the scanning row, but it also affects the bias across the non-scanning cells on the same column. This undesired bias may force the particles of a non-scanning cell to migrate to the opposite electrode. This results in changes in image density and a significant deterioration of the display contrast. A system having gating electrodes was disclosed in U.S. Pat. Nos. 4,655,897 and 5,177,476 (assigned to Copytele, Inc.) to provide EPDs capable of high resolution at relative high driving voltage using a two layer electrode structure, one of which layers serves as a gating electrode. Although these references teach how the threshold voltage may be raised by the use of gating electrodes, the cost for fabricating the two electrode layers is extremely high due to the complexity of the structure and the low yield rate. In addition, in this type of EPDs, the electrodes are exposed to the solvent, which could result in an undesired electroplating effect and deterioration in the display operation longevity.
The in-plane switched EPD device disclosed in U.S. Pat. No. 6,239,896 uses a magnetic bottom substrate to attract the magnetic particles and provide a threshold effect against the undesirable particle movement. The row and column electrodes are implemented on the bottom layers forming the driving dot matrix. The in-plane electrodes are significantly more difficult to manufacture than the normal up-down electrodes, particularly for high resolution displays. The switching rate of the in-plane displays are slower at a comparable operation voltage since the distance between electrodes in the in-plane switching mode is typically larger than the normal up-down mode. Moreover, the color saturation of a color display will be poor due to the lack of either true white or true black state.
Therefore, there is still a need for an electrophoretic display in which the cross talk and cross bias effects will not cause a degradation of display performance, even if cells having a relatively low intrinsic threshold voltage are used.